The present invention relates to transposed multi-strand conductors for use in an electric rotary machine such as a turbine generator, and more particularly to transposed multi-strand conductors suitable for windings in an armature in which a length of conductor portion received in the slot is short and the number of strands is large.
In a large capacity generator such as a turbine generator, a number of axially extending slots are formed in the inner periphery of a laminated fixed iron core and multi-strand conductors or bars for constituting armature windings are received in the slots, the armature bars being connected with each other at their opposite end portions externally projecting out of the core slots.
When an A.C. current flows in such a multi-strand bar, leakage magnetic flux circumferentially intersecting the slot is generated and voltages are induced across strands at portions in the longitudinal direction of the armature bar. If, in any pair of strands, a large difference is produced between the induced voltages across the strands over the length thereof, large circulating currents flow in the strands in the form of a closed loop, resulting in increase in losses as well as in heat generated in the armature bar. To cope with this problem, a method has been proposed in which the respective strands in the armature bar are transposed in various manners so as to make substantially uniform the voltages induced in the strands of the bar over the length thereof to prevent such circulating currents from flowing in the strands, as disclosed, for example, in U.S. Pat. Nos. 3,118,015 and 3,188,377.
The transposition of each of the strands in an armature bar is performed by successively changing the positions of the respective strands in the bar. Assume that each of the strands is circumferentially successively transposed in the cross-section of the bar about the center of the cross-section, and that the angle of rotation of each strand represents the extent of transposition of the strand. For example, "360.degree. transposition" represents the transposition that each of the strands of an armature bar starting from a point at one end of a slot, in which the bar is received, is transposed to pass through various circumferential positions in the cross-section of the bar to come to the same position at the other end of the slot as the starting position, when viewed in the cross-section of the bar.
In an ordinary generator, all the strands of an armature bar are shorted at their opposite ends with each other. Since there exists leakage flux at the opposite end portions of the generator, voltages are induced due to such leakage flux at the end portions of the armature bar so that circulating currents flow in the bar to cause losses in the form of heat. Such losses may be decreased by correctly reversing the respective positions of the strands at the opposite ends of the bar to each other so as to make opposite the respective polarities of the voltages induced across the same strands at their opposite ends to thereby cancel the induced voltages with each other.
FIG. 1 shows an example of an armature bar 1 which is subjected to the Roebel transposition in which the angle of transposition is 540.degree., that is one rotation and a half. The armature bar 1 comprises a straight portion 2 of twisted strands adapted to fit in a winding slot of an electric rotary machine (not shown) and also includes opposite end-turn portions 3 and 4 (only portions are shown) which curve both circumferentially and radially from the straight slot portion 2, along a complex curve. The armature bar 1 is made up of two stacks of 5 and 6 of strands, with each stack containing six strands, therefore twelve strands a to l being contained in the armature bar 1. The transposition is accomplished by respectively bending the strands from the top of one stack 6 into the other stack 5 while the strands from the other stack 5 are bent from the bottom thereof back into the one stack 6. The transposed positions of the respective strands a to l are designated with the same lower case letter designations a to l of the corresponding strands. The transposition pitch P.sub.1 in the vicinity of the opposite ends of the slot-inside bar portion 2 is selected to be about a half of the transposition pitch P.sub.2 in the vicinity of the center portion of the same. By selecting the transposition pitch in this manner, the inductance provided in the respective strands in the slot may be made substantially uniform. Since the number of transpositions from one stack to the next one is three for each strand in the case of 540.degree. transposition, the total number of transpositions of each bar in one slot is 3n, where n represents the number of all the strands of the bar. FIG. 2 shows the state of the strand transposition from one stack to the next.
In such a case as above where the number of transpositions is large and the pitch of transposition is short, the possibility that the thin strand insulation is damaged to thereby cause shortcircuit in the strands increases.
In order to increase the output density of a generator, that is, in order to reduce the generator volume per unit capacity thereof, generally a method is employed in which the axial length of the generator is reduced without so changing the dimension of the cross-section in the direction perpendicular to the axial direction of the generator. Accordingly, the axial length of the generator is made shorter as the density of output of the generator is made higher and in the case where the number of strands is large the above-mentioned possibility of occurrence of strand shortcircuit greatly increases.
As a result, in the case where the 540.degree. transposition can not be performed, it is necessary to employ the 450.degree. transposition, that is the transposition of one rotation and a quarter, or the 360.degree. transposition, that is one rotation. The state of transposition of strands when the 360.degree. transposition is employed is briefly shown in FIG. 3 by using two strands a and d representing all the strands. In FIG. 3, the solid line indicates that the strand exists in the stack 5 on this side of the plane of the paper, while the broken line indicates that the strand exists in the stack 6 on the rear side of the paper plane. The currents I.sub.1 to I.sub.4 induced in the strands a and d at their opposite end portions outside the slot due to the slot end magnetic flux .0..sub.1, .0..sub.2 flow in the direction to add with each other because the respective strand is arranged in the same positions at its opposite end portions in the 360.degree. transposition so that a large circulating current flows in the pair of strands which are shorted at their opposite end portions to form a closed loop, resulting in a possibility of generating local overheat.
Further, the Roebel transposition method has been proposed in which the number of strands is small and the arrangement of the strands at their opposite end portions are reversed to each other. An example of this transposition method is shown in FIG. 4 and the state that each strand shifts to the next stack is shown in FIG. 5. The bar 1 is transposed by 180.degree., that is by half rotation, at the straight slot-inside bar portion 2. Accordingly, in the case of this transposition, the currents induced in the respective strands at their opposite slot-outside end portions due to the slot end magnetic flux flow in the direction to cancel with each other so that the circulating current flowing in the pair of strands in the form of a closed loop is small. However, the inductance value of the respective strands can not be made uniform so that the distribution of currents flowing in the respective strands may greatly vary as shown in FIG. 6, resulting in increase of the losses in the form of heat generated in the bar so as to cause a possibility of occurence of local overheating.